A video compression technology such as MPEG2, MPEG4, H.264 and the like normally supports a single layer video bitstream encoding for video services. Before the video encoding is proceeded, major factors for the video services including resolutions, FPS (Frame per Second) and image qualities should be predetermined according to terminal device performances, viewer ratings, network environments and the like.
Such a video compression should be performed respectively for all the video contents, depending on their respective encoding factors, so as to make a streaming service suitable for all the various device performances and service demands. Since, however, it is problematic in view of efficiency, it has been supposed to place a transcoder between a streaming server and a client terminal for supporting the transformation of video contents. Such a video transformation at a mid node is actually effective in reducing the necessary number of encoding steps for the streaming server of a head end, but there is also a drawback that it should be accompanied with an additional decoding at the mid node due to the fundamental problem of the single layer based video compression technology, which may result in a defective performance of the service. Because the additional burden due to such a mid node transformation seems unsolvable, a root level improvement is required to fundamentally solve the problem.
For example, there is a fatal security drawback that a full or a selective encryption of prior arts may be useless to secure data when the aforementioned technology is employed because a decryption that possibly causes a fatal security error is essentially necessary for implementing the aforementioned technology, in which an original video data obtained during the process of decoding at the transcoder, used as a mid nod for video transformation, should be encoded again.
In recent years, a multi layer SVC technology capable of forming a multi layer video has been developed, and the standardization thereof was completed in 2007 by ITU-T (International Telecommunication Union Telecommunication). This compression technology is different from the aforementioned single layer SVC technology in that the video image is composed of a base layer and enhancement layers, wherein the base layer contains a core information of the video data and can be independently decoded for playback whereas each enhancement layer contains only an auxiliary information for enhancing the quality of video images and should be accompanied with the base layer for video services because an independent decoding thereof is meaningless. Due to such a configuration, it is possible to satisfy various service demands in spite of applying only a single encoding. For example, if unnecessary portions of the enhancing layers are removed by using an extractor, it is possible to transmit a video stream suitable for network bandwidth and device performances such as a screen size, a network performance, and a decoding speed.
This advantageous configuration serves as a technical base of the selective encryption, in which even an encrypted video data can be transformed into a desired video format without a decryption process at a mid node including the extractor.
However, such selective encryption technologies of prior arts are mainly focused on an encryption for a single layer based video compression technology, and also the evaluation technique of a selective encryption for SVC is limited to measuring a distortion factor resulted from the encryption whereas a truly necessary cryptographic strength is not evaluated as it should be. Further, in case of a commercial streaming service, it must be necessary to have a criterion for deciding which methodology among the prior art encryption technologies would be better for securing data such as to provide reliable and safe services to a respective user, but research and development thereof has not been properly proceeded hitherto.
Recently, an advanced media service like IPTV faces big issues such as a safe retransmission and a private storing of the media data, and for supporting them it is necessary to suitably adapt cryptographic strengths according to service demands while transforming the media data.
Further, the video data encryption is different from a text based data encryption in that a huge data is encrypted and the encryption of specific bits is restricted to comply with a respective video coding format. Accordingly, it is necessary to consider which part of the video data is desired to secure with which encryption algorithm. If an encryption does not comply with a corresponding video format, the following decoding thereof will be impossible.
The aforementioned video distortion by encryption has been generally used to prevent an unauthorized user from viewing the video image, and is now drawing more attentions as a means for inducing users to subscribe for commercial or supplementary viewing channels and programs by providing a time-limited broadcast or a partially exposed broadcast. In addition, it can be used to enable a safe automatic differentiation or differentiation of the streaming service by providing a streaming image of suitable type and quality corresponding to viewer's subscription type and rating.
In order to overcome the aforementioned drawbacks, it is necessary to develop and provide an adaptive policy model capable of providing reliable and safe streaming services by securing the SVC video image most safely, differentially, and effectively in respect of a cryptographic strength, a distortion factor, and a decoding performance, according to major service demands for users such as network environments, service ratings, device performance, and contents retransmission.